Galileo Galilei discovered that the density of a liquid changes with temperature. He quickly realized this phenomenon could be harnessed to create an instrument for measuring ambient temperature, and that is how the so-called “Galileo thermometer” was born. Glass balls float in a liquid at a height that depends on their density. As this property of the medium changes, the spheres move and indicate the current temperature. We tell you the details of how this strange and ancient instrument works.
There are many ways to measure temperature. We can buy a mercury thermometer, count a cricket’s chirps, or use the Galileo thermometer, an instrument for measuring atmospheric temperature invented by Galileo Galilei about 400 years ago. This famous physicist discovered that the density of liquids changes with temperature, which – according to Archimedes’ principle – changes their buoyancy. Taking advantage of this phenomenon, Galileo built a thermometer: a glass cylinder filled with alcohol, in which a series of glass balls filled with colored liquid are suspended.
The color of this liquid only serves to make it easier to identify. Since each of these spheres has a different amount of liquid inside, their “density” is different, and they “float” at different heights within the alcohol. Each one has a tag (attached or hanging) indicating a temperature. Unlike mercury thermometers, the main tube is not graduated.
When the ambient temperature changes, the alcohol changes its own temperature and varies its density. The spheres rearrange, floating at a new height determined by their own density. When thermal equilibrium is reached, two groups of spheres generally form – one at the bottom of the cylinder and the other at the top. The current temperature is that indicated by the sphere floating at the lowest height within the upper group. It seems complicated, but it is actually very simple to use.
The question you are surely asking yourself is: Why don’t you see thermometers like these everywhere? And the answer has to do with their poor operating range. They are basically used as decoration, and the most common models only have 5 or 6 spheres, which can indicate temperatures between 18 and 26 degrees Celsius. If it is colder or hotter, the Galileo thermometer cannot tell you. Nor can you know if it is 20, 20.2 or 20.5 degrees; it only tells you that the current temperature “around 20 degrees”. If we add that its cost is hundreds of times higher than that of a mercury thermometer, we understand why they are so uncommon.
Of course, none of its imperfections takes away credit from Galileo Galilei, who in an era when almost everything that happened was attributed to magic or the dark wishes of some more or less powerful god, discovered a physical principle and was able to turn it into a useful instrument. Don’t you think?
